Fixation of biological specimens by high hydrostatic pressure

We investigated whether or not high hydrostatic pressure could be used as a new method of histological fixation. Various high hydrostatic pressures from 200 to 800 MPa (Mega Pascal) were applied to fresh organs including heart, liver, intestine and kidney of mice, and the gross and light microscopic findings were compared with those of organs fixed with formalin and those without fixation. The specimens processed with pressure were firm, elastic and pale, somewhat resembling the formalin fixed ones. Light microscopic examination revealed well preserved fine structures in the heart and intestine subjected to approximately 450 MPa for 15 min at room temperature, although the fine structures of the liver and kidney were not well preserved. The enzyme activity of alkaline phosphatase was rarely preserved in the formalin-fixed kidney, but could be easily detected in kidneys processed with a pressure of 450 MPa. We conclude that high hydrostatic pressure can be used as a fixative, both for preserving the basic tissue structure in certain organs and for retaining the biological activity of an enzyme. In addition, the fixation is uniform, the processing is fast, and the materials can be handled safely. At present, however, its application is limited to specific organs.